ghc-9.2.1: GHC/HsToCore/Match/Constructor.hs
{-# LANGUAGE CPP #-}
{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
Pattern-matching constructors
-}
module GHC.HsToCore.Match.Constructor ( matchConFamily, matchPatSyn ) where
#include "HsVersions.h"
import GHC.Prelude
import {-# SOURCE #-} GHC.HsToCore.Match ( match )
import GHC.Hs
import GHC.HsToCore.Binds
import GHC.Core.ConLike
import GHC.Types.Basic ( Origin(..) )
import GHC.Tc.Utils.TcType
import GHC.Core.Multiplicity
import GHC.HsToCore.Monad
import GHC.HsToCore.Utils
import GHC.Core ( CoreExpr )
import GHC.Core.Make ( mkCoreLets )
import GHC.Utils.Misc
import GHC.Types.Id
import GHC.Types.Name.Env
import GHC.Types.FieldLabel ( flSelector )
import GHC.Types.SrcLoc
import GHC.Utils.Outputable
import GHC.Utils.Panic
import Control.Monad(liftM)
import Data.List (groupBy)
import Data.List.NonEmpty (NonEmpty(..))
{-
We are confronted with the first column of patterns in a set of
equations, all beginning with constructors from one ``family'' (e.g.,
@[]@ and @:@ make up the @List@ ``family''). We want to generate the
alternatives for a @Case@ expression. There are several choices:
\begin{enumerate}
\item
Generate an alternative for every constructor in the family, whether
they are used in this set of equations or not; this is what the Wadler
chapter does.
\begin{description}
\item[Advantages:]
(a)~Simple. (b)~It may also be that large sparsely-used constructor
families are mainly handled by the code for literals.
\item[Disadvantages:]
(a)~Not practical for large sparsely-used constructor families, e.g.,
the ASCII character set. (b)~Have to look up a list of what
constructors make up the whole family.
\end{description}
\item
Generate an alternative for each constructor used, then add a default
alternative in case some constructors in the family weren't used.
\begin{description}
\item[Advantages:]
(a)~Alternatives aren't generated for unused constructors. (b)~The
STG is quite happy with defaults. (c)~No lookup in an environment needed.
\item[Disadvantages:]
(a)~A spurious default alternative may be generated.
\end{description}
\item
``Do it right:'' generate an alternative for each constructor used,
and add a default alternative if all constructors in the family
weren't used.
\begin{description}
\item[Advantages:]
(a)~You will get cases with only one alternative (and no default),
which should be amenable to optimisation. Tuples are a common example.
\item[Disadvantages:]
(b)~Have to look up constructor families in TDE (as above).
\end{description}
\end{enumerate}
We are implementing the ``do-it-right'' option for now. The arguments
to @matchConFamily@ are the same as to @match@; the extra @Int@
returned is the number of constructors in the family.
The function @matchConFamily@ is concerned with this
have-we-used-all-the-constructors? question; the local function
@match_cons_used@ does all the real work.
-}
matchConFamily :: NonEmpty Id
-> Type
-> NonEmpty (NonEmpty EquationInfo)
-> DsM (MatchResult CoreExpr)
-- Each group of eqns is for a single constructor
matchConFamily (var :| vars) ty groups
= do let mult = idMult var
-- Each variable in the argument list correspond to one column in the
-- pattern matching equations. Its multiplicity is the context
-- multiplicity of the pattern. We extract that multiplicity, so that
-- 'matchOneconLike' knows the context multiplicity, in case it needs
-- to come up with new variables.
alts <- mapM (fmap toRealAlt . matchOneConLike vars ty mult) groups
return (mkCoAlgCaseMatchResult var ty alts)
where
toRealAlt alt = case alt_pat alt of
RealDataCon dcon -> alt{ alt_pat = dcon }
_ -> panic "matchConFamily: not RealDataCon"
matchPatSyn :: NonEmpty Id
-> Type
-> NonEmpty EquationInfo
-> DsM (MatchResult CoreExpr)
matchPatSyn (var :| vars) ty eqns
= do let mult = idMult var
alt <- fmap toSynAlt $ matchOneConLike vars ty mult eqns
return (mkCoSynCaseMatchResult var ty alt)
where
toSynAlt alt = case alt_pat alt of
PatSynCon psyn -> alt{ alt_pat = psyn }
_ -> panic "matchPatSyn: not PatSynCon"
type ConArgPats = HsConPatDetails GhcTc
matchOneConLike :: [Id]
-> Type
-> Mult
-> NonEmpty EquationInfo
-> DsM (CaseAlt ConLike)
matchOneConLike vars ty mult (eqn1 :| eqns) -- All eqns for a single constructor
= do { let inst_tys = ASSERT( all tcIsTcTyVar ex_tvs )
-- ex_tvs can only be tyvars as data types in source
-- Haskell cannot mention covar yet (Aug 2018).
ASSERT( tvs1 `equalLength` ex_tvs )
arg_tys ++ mkTyVarTys tvs1
val_arg_tys = conLikeInstOrigArgTys con1 inst_tys
-- dataConInstOrigArgTys takes the univ and existential tyvars
-- and returns the types of the *value* args, which is what we want
match_group :: [Id]
-> [(ConArgPats, EquationInfo)] -> DsM (MatchResult CoreExpr)
-- All members of the group have compatible ConArgPats
match_group arg_vars arg_eqn_prs
= ASSERT( notNull arg_eqn_prs )
do { (wraps, eqns') <- liftM unzip (mapM shift arg_eqn_prs)
; let group_arg_vars = select_arg_vars arg_vars arg_eqn_prs
; match_result <- match (group_arg_vars ++ vars) ty eqns'
; return $ foldr1 (.) wraps <$> match_result
}
shift (_, eqn@(EqnInfo
{ eqn_pats = ConPat
{ pat_args = args
, pat_con_ext = ConPatTc
{ cpt_tvs = tvs
, cpt_dicts = ds
, cpt_binds = bind
}
} : pats
}))
= do ds_bind <- dsTcEvBinds bind
return ( wrapBinds (tvs `zip` tvs1)
. wrapBinds (ds `zip` dicts1)
. mkCoreLets ds_bind
, eqn { eqn_orig = Generated
, eqn_pats = conArgPats val_arg_tys args ++ pats }
)
shift (_, (EqnInfo { eqn_pats = ps })) = pprPanic "matchOneCon/shift" (ppr ps)
; let scaled_arg_tys = map (scaleScaled mult) val_arg_tys
-- The 'val_arg_tys' are taken from the data type definition, they
-- do not take into account the context multiplicity, therefore we
-- need to scale them back to get the correct context multiplicity
-- to desugar the sub-pattern in each field. We need to know these
-- multiplicity because of the invariant that, in Core, binders in a
-- constructor pattern must be scaled by the multiplicity of the
-- case. See Note [Case expression invariants].
; arg_vars <- selectConMatchVars scaled_arg_tys args1
-- Use the first equation as a source of
-- suggestions for the new variables
-- Divide into sub-groups; see Note [Record patterns]
; let groups :: [[(ConArgPats, EquationInfo)]]
groups = groupBy compatible_pats [ (pat_args (firstPat eqn), eqn)
| eqn <- eqn1:eqns ]
; match_results <- mapM (match_group arg_vars) groups
; return $ MkCaseAlt{ alt_pat = con1,
alt_bndrs = tvs1 ++ dicts1 ++ arg_vars,
alt_wrapper = wrapper1,
alt_result = foldr1 combineMatchResults match_results } }
where
ConPat { pat_con = L _ con1
, pat_args = args1
, pat_con_ext = ConPatTc
{ cpt_arg_tys = arg_tys
, cpt_wrap = wrapper1
, cpt_tvs = tvs1
, cpt_dicts = dicts1
}
} = firstPat eqn1
fields1 = map flSelector (conLikeFieldLabels con1)
ex_tvs = conLikeExTyCoVars con1
-- Choose the right arg_vars in the right order for this group
-- Note [Record patterns]
select_arg_vars :: [Id] -> [(ConArgPats, EquationInfo)] -> [Id]
select_arg_vars arg_vars ((arg_pats, _) : _)
| RecCon flds <- arg_pats
, let rpats = rec_flds flds
, not (null rpats) -- Treated specially; cf conArgPats
= ASSERT2( fields1 `equalLength` arg_vars,
ppr con1 $$ ppr fields1 $$ ppr arg_vars )
map lookup_fld rpats
| otherwise
= arg_vars
where
fld_var_env = mkNameEnv $ zipEqual "get_arg_vars" fields1 arg_vars
lookup_fld (L _ rpat) = lookupNameEnv_NF fld_var_env
(idName (unLoc (hsRecFieldId rpat)))
select_arg_vars _ [] = panic "matchOneCon/select_arg_vars []"
-----------------
compatible_pats :: (ConArgPats,a) -> (ConArgPats,a) -> Bool
-- Two constructors have compatible argument patterns if the number
-- and order of sub-matches is the same in both cases
compatible_pats (RecCon flds1, _) (RecCon flds2, _) = same_fields flds1 flds2
compatible_pats (RecCon flds1, _) _ = null (rec_flds flds1)
compatible_pats _ (RecCon flds2, _) = null (rec_flds flds2)
compatible_pats _ _ = True -- Prefix or infix con
same_fields :: HsRecFields GhcTc (LPat GhcTc) -> HsRecFields GhcTc (LPat GhcTc)
-> Bool
same_fields flds1 flds2
= all2 (\(L _ f1) (L _ f2)
-> unLoc (hsRecFieldId f1) == unLoc (hsRecFieldId f2))
(rec_flds flds1) (rec_flds flds2)
-----------------
selectConMatchVars :: [Scaled Type] -> ConArgPats -> DsM [Id]
selectConMatchVars arg_tys con = case con of
(RecCon {}) -> newSysLocalsDsNoLP arg_tys
(PrefixCon _ ps) -> selectMatchVars (zipMults arg_tys ps)
(InfixCon p1 p2) -> selectMatchVars (zipMults arg_tys [p1, p2])
where
zipMults = zipWithEqual "selectConMatchVar" (\a b -> (scaledMult a, unLoc b))
conArgPats :: [Scaled Type]-- Instantiated argument types
-- Used only to fill in the types of WildPats, which
-- are probably never looked at anyway
-> ConArgPats
-> [Pat GhcTc]
conArgPats _arg_tys (PrefixCon _ ps) = map unLoc ps
conArgPats _arg_tys (InfixCon p1 p2) = [unLoc p1, unLoc p2]
conArgPats arg_tys (RecCon (HsRecFields { rec_flds = rpats }))
| null rpats = map WildPat (map scaledThing arg_tys)
-- Important special case for C {}, which can be used for a
-- datacon that isn't declared to have fields at all
| otherwise = map (unLoc . hsRecFieldArg . unLoc) rpats
{-
Note [Record patterns]
~~~~~~~~~~~~~~~~~~~~~~
Consider
data T = T { x,y,z :: Bool }
f (T { y=True, x=False }) = ...
We must match the patterns IN THE ORDER GIVEN, thus for the first
one we match y=True before x=False. See #246; or imagine
matching against (T { y=False, x=undefined }): should fail without
touching the undefined.
Now consider:
f (T { y=True, x=False }) = ...
f (T { x=True, y= False}) = ...
In the first we must test y first; in the second we must test x
first. So we must divide even the equations for a single constructor
T into sub-groups, based on whether they match the same field in the
same order. That's what the (groupBy compatible_pats) grouping.
All non-record patterns are "compatible" in this sense, because the
positional patterns (T a b) and (a `T` b) all match the arguments
in order. Also T {} is special because it's equivalent to (T _ _).
Hence the (null rpats) checks here and there.
-}